iTSSe TSS ADVANCED MATERIALS & PROCESSES | MARCH 2026 45 iTSSe TSS In suspension plasma spraying (SPS), fine particles in the nanometer-range enable the formation of specific microstructures (e.g., dense, porous, columnar). Due to their low flowability, such fine particles have the potential to cause blockage within the spray system and are consequently combined with a liquid. The overall process involves atomization, interaction with the plasma plume, and the formation of droplets and agglomerates[1,2]. However, crucial process parameters such as liquid type (water or ethanol), limited mass fraction, and particle size distribution must be considered for the resulting coatings[3]. A related study focused on degradation of the SPS suspension before spraying[4]. Degradation in the form of agglomeration of solids and forming larger particles upon long-term storage was identified and quantified using diagnostics and coating investigation. This article examines the effect of such a degraded suspension on the resulting coating properties. EXPERIMENTAL METHODS Setup. The setup for all experiments was a complete Axial III plasma generator (Northwest Mettech Corp., Surrey, BC, Canada) with a converging-diverging nozzle with a throat diameter of 7.94 mm (5/16 in.). A Treibacher AuerCoat Y2O3 30 wt% T1 (SUS 633 T, water-based) was used as suspension. The recommended expiration date of the suspension was six months. The first set of experiments (Batch 1) was sprayed six months after suspension delivery. A second set was sprayed after 18 months (Batch 2). Additional particle size analyses were conducted 12 months after delivery (Intermediate State). The composition of the DEGRADATION OF SUSPENSIONS FOR PLASMA SPRAYING: A TECHNICAL NOTE This study describes how larger particle size and the hereby resulting speckle formation can affect coating properties and decrease deposition efficiency. Johannes-Christian Schmitt, Institute of Energy Materials and Devices (IMD-2), Forschungszentrum Jülich GmbH, Jülich, Germany Georg Mauer,* IMD-2 and Department of Mechanical Engineering, TU Dortmund University, Dortmund, Germany plasma gas is provided in Table 1[4]. The suspension feed rate was set at 45 ml/min in each case. The spray distance was 100 mm and the surface scanning speed of the robot was 1500 mm/s. Based on preliminary experiments, a 1 s break was set between each spray meander for a total number of 20 spray passes. Spraying was performed on stainless steel substrates that had previously been grit blasted with F36 (Al2O3, 425-600 µm). All samples were cooled by compressed air on the backside[4]. Particle Diagnostics. Particle diagnostics were performed using the Accuraspray 4.0 (TECNAR Automation Ltd., St-Bruno, QC, Canada). This device is an in-line thermal spray sensor that measures the particle temperature and velocity, as well as dimension, orientation, intensity, and stability of the spray plume. The measurement is based on an ensemble method that provides averaged values, where the particle velocity and temperature are obtained from the intensity maximum of the particle plume over a period of 60 s. A more detailed explanation of the measurement principle is given elsewhere[5]. Characterization Methods. The particle size distribution was measured by laser diffractometry (Horiba LA950, Retsch Technology GmbH, Haan, Germany). To evaluate the deposition efficiency (DE), each sample was weighed before and after the deposition. The thickness of the substrate and the as-sprayed specimens were evaluated by using a micrometer. Topography of the coatings was analyzed using an optical profilometer with a CHR10000 sensor (model CT350T, cyberTECHNOLOGIES GmbH, Germany). The roughness measurements of all coatings were carried out with the same device and a CHR1000 sensor, which provides a higher resolution. Here, the sample surface was scanned over five line profiles in the horizontal and vertical directions, each 20 mm in length and an offset of 1 mm, with a step size of 1 µm. *Member of ASM International FEATURE TABLE 1 — APPLIED GAS MIXTURES IN THIS STUDY Gas Ar N2 H2 Total Fraction 25% 60% 15% 220 slpm 55 slpm 132 slpm 33 slpm Note: Current was set to I = 220°A for each generator[4].
RkJQdWJsaXNoZXIy MTYyMzk3NQ==